Autoscroll new background tabs on iPad

ios/chrome/browser/ui/tabs/tab_strip_controller.mm

Index: ios/chrome/browser/ui/tabs/tab_strip_controller.mm
diff --git a/ios/chrome/browser/ui/tabs/tab_strip_controller.mm b/ios/chrome/browser/ui/tabs/tab_strip_controller.mm
index ced59b8ddf92bb3c4f5b09453dda550999c6260b..215af54a90e639e31cd087aa2b128004ee277eef 100644
--- a/ios/chrome/browser/ui/tabs/tab_strip_controller.mm
+++ b/ios/chrome/browser/ui/tabs/tab_strip_controller.mm
@@ -1253,6 +1253,29 @@ - (CGFloat)minTabWidth {
 - (void)updateContentOffsetForTabIndex:(NSUInteger)tabIndex {
   DCHECK_NE(NSNotFound, static_cast<NSInteger>(tabIndex));
 
+  if (experimental_flags::IsTabStripAutoScrollNewTabsEnabled()) {
+    NSUInteger numTabsToLeft = 0;

[rohitrao (ping after 24h), 2017/01/06 15:35:21]

This could use a big comment explaining the goal of this code and how the
algorithm works.

[liaoyuke, 2017/01/06 20:46:49]

Done.

+    NSUInteger i = 0;
+    for (TabView* tab in _tabArray.get()) {

[rohitrao (ping after 24h), 2017/01/06 15:35:21]

What is this for loop trying to do?  I think can be replaced with:

[self indexForModelIndex:tabIndex]

[liaoyuke, 2017/01/06 20:46:49]

To find the number of non-closing tabs to the left of the current tab. It seems
to be different from indexForModelIndex. For example, say the index of current
tab is 3, and tab of index 0 is closing. So, in this case, indexForModelIndex
would return 4, while what we need is 2.

+      if ([_closingTabs containsObject:tab])
+        ++i;
+
+      if (i == tabIndex)
+        break;
+
+      ++numTabsToLeft;
+      ++i;
+    }
+
+    const CGFloat tabHeight = CGRectGetHeight([_tabStripView bounds]);
+    CGRect scrollRect =
+        CGRectMake(_currentTabWidth * numTabsToLeft -
+                       ([self tabOverlap] * (numTabsToLeft - 1)),
+                   0, _currentTabWidth, tabHeight);
+    [_tabStripView scrollRectToVisible:scrollRect animated:YES];
+    return;
+  }
+
   if (IsCompactTablet()) {
     if (tabIndex == [_tabArray count] - 1) {
       const CGFloat tabStripAvailableSpace =